Dual waveband temperature detector

ABSTRACT

There are many industrial applications in which non-contact temperature sensing is useful for increasing production speed and quality, such as printing, laminating, extrusion, and metal forming. Disclosed is a non-contact temperature determining apparatus which uses two wide wavelength bands integrating sensors to determine the radiance ratio of a target and thereby determine a corresponding temperature of the target. Also disclosed is a non-contact temperature determining apparatus in which a beam splitter passes one wide wavelength band to a sensor and reflects another distinct wide wavelength band to another sensor from which temperature can be determined. A disclosed embodiment of the dual waveband temperature detector improves upon traditional and currently available ratio pyrometers by further reducing the cost of the system, making installation and use easier, and improving temperature detection for low temperature industrial applications.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/351,882 filed Jan. 17, 2012, which is a continuation-in-part of U.S.application Ser. No. 13/151,900, filed Jun. 2, 2011, which claims thebenefit of U.S. Provisional Application No. 61/397,077, filed on Jun. 7,2010. The entire teachings of the above applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

A pyrometer is a device that intercepts and measures thermal radiationin a non-contact temperature sensing process known as pyrometry. Atypical pyrometer has an optical system and detector; the optical systemfocuses thermal radiation onto the detector. The detector produces anoutput signal, typically a voltage, which is related to thermalradiation, or iridescence, of a target object. Therefore, the outputsignal of the detector can be used to infer the temperature of thetarget object, while negating the need for direct contact between thetemperature detecting device and the target object.

The thermal radiation of an object depends on its emissivity. Emissivityis the property of a material's surface that describes its “efficiency”at emitting thermal radiation. An emissivity value of 1.0 representsthermal radiation emission at 100% while an emissivity of 0 describesthermal radiation emission at 0% (or perfect reflection).

Typically, for non-metals and coated metals emissivity is very high, 0.8or greater, and variations in emissivity are less of a problem fornon-contact temperature detection. For example, a production process inwhich a non-metallic material with an emissivity of 0.9 is to betemperature-controlled, and if normal material variations causeemissivity variations of ±0.01, the associated temperature error will beof the order of 0.01 divided by 0.9, or about 1% of the temperaturereading, an acceptable variation for many applications. In contrast, fora production process in which the temperature of a metal having anemissivity of 0.2 must be controlled, emissivity variations of ±0.01will produce an error on the order of 0.01 divided by 0.2, or about 5%of the temperature reading, which is typically unacceptable.Additionally, metal finishes, which play a significant role inemissivity, tend to have more variations than non-metals. A commonproblem is aluminum because its emissivity is low and variable due toalloying, surface oxidation, variations in surface finish, and otherfactors.

Ratio pyrometers typically use two photo sensors to detect radiation attwo separate wavebands. Temperature can be determined by taking theratio of the detected radiation. Traditional ratio pyrometers, whichdate from about the middle of the last century, operate with two narrowspectral bands. As such, they are successful at measuring targets ofstrong radiance. In other words, they are useful for measuring hightemperature targets. However, for targets with lower temperatures, andtherefore a lower radiance, the narrow spectral bands receiveinsufficient photo signals, rendering the technique useless.

U.S. Pat. No. 5,764,684, titled “Infrared Thermocouple Improvements,”issued Jun. 9, 1998, discloses a device and method which employsinfrared sensors with very wide bandwidths, thereby increasing theradiation detector output, such that relatively low temperatures (i.e.,less than 50° Celsius (C.)) can be measured. Further disclosed is theside-by-side placement of a short wavelength low emissivity infrared(IR) thermocouple and a long wavelength infrared thermal couple focusedon the same target area at a particular distance. The two thermocoupleinput channels provide input to a computer or PLC. The computer or PLChas the computational ability to solve two equations with two unknowns,with one such solution being the computation of differentials in signalrelative to the initial calibration. This solution depends on theassumption that the emissivity ratio remains constant for the twowavebands of interest.

SUMMARY OF THE INVENTION

An embodiment of the presented dual waveband temperature detectorimproves upon conventional two-color pyrometry systems by furtherreducing system costs, improving ease of installation and use, andparticularly for improving temperature detection for low temperatureindustrial applications (i.e., temperatures less than 500° F. (F.) or260° C. (C.)), which has not been practical for conventional two-colorpyrometry.

Prior art two-color pyrometry systems used a side-by-side configurationof a short wavelength low emissivity IR thermocouple and a longwavelength IR thermocouple, where the low emissivity IR thermocouple anda long wavelength IR thermal couple are both focused on the same targetarea at a particular distance. Such a configuration can cause inherentengineering difficulties, such as parallax error, increasing thedifficulty of the installation and use of the device. An embodiment ofthe presented dual waveband temperature detector uses a beam splitter tosuperimpose the optical axes of both sensors; eliminating parallaxerrors and improving ease of installation and use.

Another feature of an example embodiment of the dual wavebandtemperature detector enables non-contact detection of low temperaturetargets and reduces two-color pyrometer costs. The use of wide bandwidthcomponents enables the sensors to capture more thermal energy thantraditional narrow bandwidth two-color pyrometers. By enabling moreenergy to be captured with a wide range of wavelengths, which includelonger wavelength energy, lower energy targets (i.e., lower temperatureand longer wavelength emitting targets) can be accurately detected.Further, by enabling more energy to be captured, less costly optical andelectronic components can be used without sacrificing performance.

According to an example embodiment of a temperature detector, a dualwaveband temperature detector apparatus, and corresponding method fornon-contact temperature sensing, includes a beam splitter, first andsecond sensors, and electronics to provide an output representative ofthe detected surface temperature of a target.

A further example embodiment of a radiation detector includes first andsecond wideband integrating sensors, and electronics to compute theratio of the integrated signals and provide an output. The output can berepresentative of the detected surface temperature of a target.

A further alternative example embodiment includes a beam splitter, firstand second radiation integrating sensors, and electronics to provide anoutput representative of the detected surface temperature of a target.

The beam splitter can be made of sapphire, which has transmissioncharacteristics in which wavelengths of less than about 5 microns (μm)are passed through and wavelengths of greater than about 5 μm arereflected. The first radiation integrating sensor can cover a widewavelength range including less than or equal to about 5 μm, and thesecond integrating sensor can cover a wide wavelength range includinggreater than or equal to about 5 μm.

The electronics can compute a ratio of the detected radiation, which canindicate the temperature of a target. The window of the first sensor canbe made of the same material as the beam splitter. The first and secondradiation integrating sensors can be thermopiles configured so that theyshare a common heat sink, which can be formed of aluminum, to maintainthe cold junctions at a common temperature. The dual wavebandtemperature detector can further include a lens, which can be a Fresnellens.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a high level overview of an application of an embodiment ofthe present invention dual waveband temperature detector;

FIGS. 2A and 2B are high level schematic drawings of example embodimentsof the dual-wave temperature detector;

FIG. 3 is a high level schematic drawing of the circuitry of an exampleembodiment of a dual waveband temperature detector;

FIG. 4 is a high level diagram of a test setup used to test an exampleembodiment of a dual waveband temperature detector;

FIGS. 5A and 5B are measured test data of an example embodiment of adual waveband temperature detector;

FIGS. 6A and 6B are radiance ratios computed from measured test data ofan example embodiment of a dual waveband temperature detector;

FIGS. 7A and 7B are emissivity ratios computed from measured test dataof an example embodiment of a dual waveband temperature detector;

FIG. 8 is a plot of the Plank function of radiated energy as a functionof wavelength; and or

FIG. 9 is a plot of the optical transmission of a 1 millimeter thickplate of sapphire.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows.

The invention in general relates to pyrometry and more particularly totwo-color pyrometry.

There are many applications in which non-contact temperature sensing isuseful, such as for increasing production speed and quality ofindustrial processes. Example industrial applications include printing,laminating, extrusion, and metal forming. Traditional infrared (IR)temperature detection methods are difficult to implement for manyapplications because of inherent engineering problems. However,two-color pyrometry yields significant improvements over the singlewavelength devices traditionally used for IR temperature detection, andtherefore, eliminates most of the difficulties associated with suchdevices.

Many of the drawbacks of conventional two-color pyrometry systems, suchas size, complexity, and cost, have been improved upon with the recentimprovements in the field; however, the current two-color pyrometrysystems can be further improved. An example embodiment of the dualwaveband temperature detector does so by further reducing the cost ofthe system, making installation and use easier, and improvingtemperature detection for low temperature (i.e., less than 500° F. (F.)or 260° C.) industrial applications.

Traditional ratio pyrometers measure the thermal radiance at two narrowwavelength bands and calculate the temperature from the ratio of theseradiances, particularly when the emissivity c is unknown. Such devicesare typically used for metals at high temperatures, where the emittedenergy is intense and at short wavelengths, often in the visible range.Typically, wavelength bandwidths ∂λ/λ less than or equal to 0.01 areconsidered to be narrow bandwidth, while those equal to or greater than0.1 are considered to be wide bandwidth.

An embodiment of the current invention, by utilizing wide wavelengthbands, can further reduce the cost of ratio pyrometry systems whileaccurately monitoring a range of low temperatures for industrialapplications. Using wide wavelength bands enables the sensors to capturemore energy than narrow wavelength bands sensors. Because more energy iscapture by the wide waveband sensors, the requirements for front-endelectronics, such as amplifiers and analog-to-digital (A/D) converters,can be reduced. This enables a reduction in the cost of the apparatus.Furthermore, by utilizing wide wavelength bands, rather than narrowwavelength bands, it is possible to capture more energy with the widewavelength band sensor at longer wavelengths, and, therefore, toaccurately monitor the low temperatures typical of industrialapplications.

By integrating the energy over a wide wavelength band, the requirementthat the emissivity ratio be constant over all wavelengths is relaxed.What is required for accuracy, in an embodiment of the presentinvention, is that the ratio of the detected energy integrated over thetwo wavebands be constant, which is a more flexible requirement thanthat of maintaining a constant emissivity ratio over all wavelengths.Accordingly, the target does not have to have the properties of a “graybody,” which is the assumption that emissivity is constant as a functionof wavelength.

Many of the currently available ratio pyrometers still suffer frominherent engineering difficulties, making them less reliable and moredifficult to use than example embodiments of the dual wavebandtemperature detector presented herein. Often, these currently availableratio pyrometers have two sensors that are mounted side-by-side,configured so that both focus on one target measurement spot at oneparticular distance. At any distance other than the designed targetdistance, the optical axes of the two sensors do not focus on the samepoint, and, therefore, measure the radiance of two separate targetspots. This is called a parallax error and it limits the usefulness ofcurrent approaches. Similarly, any partial obstruction, such as smoke,dust, or moisture, of the optical field of only one of the two sensorshas the potential to cause errors because the radiance measure by thepartially obstructed sensor will be reduced.

An embodiment of the current invention superimposes both optical axes bymeans of a beam splitter (also known as a beam combiner) to always haveboth sensors aimed at the same target spot regardless of the targetdistance. Further, because the two sensors simultaneously use theidentical optical field, any partial obstruction affects both sensorsidentically. Therefore, any errors due to partial blockage areeliminated.

FIG. 1 illustrates a typical industrial application 100 in which anexample embodiment of the present invention can be used to detect andmonitor temperature. A dual wavelength temperature detector 101integrates the focused radiation of common optical axis 102 of a targetmaterial 105.

The target material 105 can have variations in emissivity. The targetmaterial 105 is shown with a low and variable emissivity ranging from0.19 to 0.22. Such emissivity variations can be due to alloying, surfaceoxidation, variations in surface finish, and other factors.

The dual wavelength detector 101 outputs a signal 110 representative ofthe surface temperature of the target material 105 via an electricalconnection to a machine user interface 111 where a user can monitor it.Electronics, such as amplifiers, A/D converters, and processors, can belocated at the dual wavelength temperature detector 101 or machine userinterface 111. The dual waveband temperature detector 101 can containtwo sensors that share a common optical axis 102. The common opticalaxis 102 can result from the superimposition of the optical axes of bothsensors.

Many applications, such as the example industrial application 100,benefit from the detection and monitoring of the temperature of a target105. For example, the quality and through-put of many industrialprocesses, such as printing, laminating, extrusion, and metal forming,can be increased by monitoring and controlling temperature so that it ismaintained at the optimal production level.

FIG. 2A is a mechanical schematic diagram of an example embodiment ofthe dual waveband temperature detector 201. The depicted dual wavebandtemperature detector 201 is comprised of: a lens 211, beam splitter 213,integrating sensors 215 a and 215 b, front-end electronics 219 a and 219b, processor 223, and enclosure 225. The external optical field 202 ofthe dual waveband temperature detector 201 is focused on a targetsurface 205 for which the temperature is to be detected. The lens 211transfers the incident radiation within the external optical field 202into the internal optical field 204, focusing it at the beam splitter213.

The beam splitter 213 separates the focused incident radiation into twoseparate and distinct wavelength bands 206 a and 206 b. The beamsplitter 213 is formed from a material, such as sapphire, with opticalproperties that enable two spectral bands of interest to be separatedand individually transferred to sensors 215 a and 215 b. Here, separatedwavelength bands 206 a and 206 b are each individually transferred tosensors 215 a and 215 b, respectively. For example, a first range ofwavelengths 206 a from about 0.5 μm to about 5 μm can be transferred tointegrating sensor 215 a, and a second range of wavelengths 206 bgreater than about 5 μm can be transferred to integrating sensor 215 b.

Sensors 215 a and 215 b each respectively integrate the incidentradiation of separated wavelength bands 206 a and 206 b, determining theradiance of each of the wavelength bands 206 a and 206 b. The outputsignals, such as a voltage representing a determined radiance, ofsensors 215 a and 215 b are individually amplified by front-endelectronics 219 a and 219 b, respectively. Each amplified determinedradiance is provided as an input to processor 223.

Processor 223 can then determine the temperature T of the target surface205 through a variety of methods, such as determining the radiance ratioof the input signals and utilizing an exponential or linear approach,and provide the temperature T as an output 210. Output 210 can be scaledelectronically to render a number equal to the target temperature.

Alternatively, the radiance ratio value can be provided to a processcontroller, such as the Eurotherm 3500 series, which has the ability toconvert the radiance ratio into a temperature by a polynomialapproximation and display that temperature in digital form. A furtheralternative is to apply the two amplified signals to a microprocessor,such as the PIC16F87XA, to linearize the radiance ratio, as a functionof temperature T, and obtain an output voltage equal to the targettemperature.

FIG. 2B illustrates a mechanical schematic diagram of an alternativeexample embodiment of the dual wavelength temperature detector 251. Thedepicted dual wavelength temperature detector 251 is comprised of: alens 261, beam splitter 263, integrating sensors 265 a and 265 b, heatsink 271, front-end electronics 269 a and 269 b, processor 273, andenclosure 275. The heat sink 271 provides a common reference temperaturefor both integrating sensors 265 a and 265 b. Any thermal conductivematerial, such as aluminum, in which the integrating sensors can beconfigured to share a common reference temperature, can be used.Although the reference temperatures for integration sensors 215 a and214 b (shown in FIG. 2A) are very likely to be the same, by providing athermal conductive material on which to configure the two integratingsensors 265 a and 265 b, the heat sink 271 is able to better ensure thata common reference temperature is provided and, therefore, a moreaccurate radiance ratio.

FIG. 3 depicts a top-level electrical schematic of an example embodimentof a dual waveband temperature detector 301. Incident radiation 304 isfiltered into two separate and distinct wavelength bands 306 a and 306b. The two incident radiation separated wavelength bands 306 a and 306 bare each individually directed to integrating sensors 315 a and 315 b,respectively. The integrating sensors 315 a and 315 b integrate therespective separated wavelength bands 306 a and 306 b of incidentradiation 304 and each produce respective output signals 307 a and 307b. Output signals 307 a and 307 b are each respectively amplified byamplifiers 321 a and 321 b to generate respective amplified signals 308a and 308 b. Amplified signals 308 a and 308 b are each digitized by A/Dconverters 322 a and 322 b, respectively. A/D converters 322 a and 322 bproduce respective digital signals 309 a and 309 b, which are providedas inputs to processor 323. Processor 323 can then use a variety ofmethods, such as determining the radiance ratio of the input signals andutilizing an exponential or linear approach, to determine thetemperature T and provide a representative output 310.

FIG. 4 illustrates an experimental test setup 400 of an embodiment of adual waveband temperature detector 401. The dual waveband temperaturedetector 401 detects the temperature of a target 405. Incident radiation402 is sensed and integrated by the dual waveband temperature detector401. The target 405 is a plate of aluminum, of which one side is paintedblack 407 and the other side is unpainted 409, sitting on top of ahot-plate heater (not shown). The integrating sensors 215 a and 215 b(as shown in FIG. 2) of the dual waveband temperature detector 401 areeach hooked up to a machine user interface 412 and 413, such as adigital liquid crystal display (LCD).

Experimental data can be measured and recorded for experimental testsetup 400 and can include data at each sample temperature such as thehot-plate heater temperature (or the control temperature), firstintegrating sensor (S1) output signal for the incident radiation ofpainted 407 and unpainted 409 aluminum, second integrating sensor (S2)output signal for the incident radiation of painted 407 and unpainted409 aluminum, and surface temperature readings of the painted 407 andunpainted 409 aluminum measured using an independent IR thermometer,such as a D501 handheld IR scanner from Exergen®. (Exergen is aregistered trademark of Exergen Corporation, 400 Pleasant St. Watertown,Mass. 02472.) The dual waveband temperature detector 401 can beconfigured so that it can be moved back and forth to detect thetemperature of the painted 407 and unpainted 409 aluminum at eachcontrol temperature.

FIGS. 5A and 5B show example experimental data measured and recorded forexperimental test setup 400 (FIG. 4).

FIG. 5A is a plot of experimental data measured and recorded for testsetup 400 and includes trend lines. FIG. 5A includes the integratedradiation measured by first sensor (S1) for both painted 407 andunpainted 409 aluminum (shown in FIG. 4), and similarly, includesintegrated radiation measured by second sensor (S2) for both painted 407and unpainted 409 aluminum. Trend lines illustrate the powerrelationship between integrated radiation and temperature T, in whichthe integrated radiation is a direct function of temperature T raised toa power.

FIG. 5B is a table of example experimental data measured and recordedfor test setup 400. The table includes measured data for: the hot-plateheater temperature, also called the control temperature (Control Temp.(° F.)); independent IR thermometer temperature readings of theunpainted 409 and painted 407 aluminum (D501 IR Thermometer (° F.));first integrating sensor (S1) output signal for the integrated incidentradiation of unpainted 409 and painted 407 aluminum in millivolts(SENSOR-1 (mV)); second integrating sensor (S2) output signal for theintegrated incident radiation of unpainted 409 and painted 407 aluminumin millivolts (SENSOR-2 (mV)).

FIGS. 6A and 6B show the calculated radiance ratio for the exampleexperimental data measured and recorded shown in FIG. 5B forexperimental test setup 400 (FIG. 4).

FIG. 6A is a plot of the ratio of integrated radiation W_(S2)/W_(S1) ofthe experimental data measured and recorded by integrating the secondand first sensors (S2 and S1, respectively) for test setup 400 andincludes trend lines. FIG. 6A includes the ratio of integrated radiationW_(S2)/W_(S1) for both painted 407 and unpainted 409 aluminum (shown inFIG. 4). Trend lines illustrate the relationship between the radianceratio and temperature T, in which radiance ratio is an exponentialfunction of temperature T.

FIG. 6B is a table of the computed ratio of integrated radiationW_(S2)/W_(S1) of the experimental data measured and recorded byintegrating the second and first sensors (S2 and S1, respectively) fortest setup 400. The ratio of integrated radiation is computed by takingthe ratio of the radiation integrated by the second sensor (S2) by theradiation integrated by the first sensor (S1) for painted 407 andunpainted 409 aluminum for each sample temperature. In other words, foreach sample of painted 407 and unpainted 409 aluminum at a controltemperature, the voltage output of S2 is divided by the voltage outputof S1.

FIG. 7A is a plot of the emissivity ratio of the painted and unpaintedaluminum calculated for each of the two integration sensors from theexample experimental data measured and recorded shown in FIG. 5B forexperimental test setup 400 (FIG. 4) and includes trend lines. Theemissivity ratio is individually computed for each sensor by taking theratio of radiances of the unpainted aluminum 409 and painted aluminum407 (shown in FIG. 4) for each sample temperature. In other words, foreach sensor at each control temperature sample, the voltage outputresulting from integrating the radiance of the unpainted aluminum 409 isdivided by the voltage output from integrating the radiance of thepainted aluminum 407 that that individual sensor.

FIG. 7B is a table of the computed emissivity ratio of the painted andunpainted aluminum calculated for each integration sensor from theexample experimental data measured and recorded shown in FIG. 5B forexperimental test setup 400.

FIGS. 7A and 7B illustrate that variations in emissivity have anegligible impact on the overall emissivity ε_(λpainted) of the painted407 and ε_(λunpainted) unpainted 409 aluminum, and that there is anegligible impact on the emissivity ratio ε_(λ2)/ε_(λ1).

FIG. 8 illustrated the Planck function thermal radiation energydistribution as a function of wavelength. In general, radiance W_(λ) isdetermined by integrating over the filter bandwidth of a pyrometer. Forwide bandwidths, such as 0.1 to 5.0 μm and 6.5 to 20 μm, the Planckintegral can be approximated by W≈εσT^(x), where x≠4, FIG. 8 illustratesthat for the wavelength band to the short wavelength side of the peakvalue of W_(λ), x>4. This is the side of the Planck function that ismore temperature sensitive. Further, for bandwidths, such as 0.1 to 5.0μm, employed at common temperatures for industrial use, the emittedenergy increases more rapidly than T⁴. Also shown in FIG. 8 is that forwavelength bands to the long wavelength side of the peak value of W_(λ),x<4. For the long wavelength side the emitted energy increases lessrapidly than T⁴.

FIG. 9 shows the optical transmission properties of sapphire, which is apreferred beam splitter material. The use of a sapphire plate for a beamsplitter exploits the optical properties of sapphire to separately sendthe two distinct spectral bands of interest to first and secondintegration sensors 215 a and 215 b (shown in FIG. 2A). FIG. 9 showsthat sapphire's internal transmission is near 100% for wavelengths 0.1-5μm. In other words, sapphire allows radiation of wavelengths 0.1-5 μm topass through it, but reflects (or blocks) radiation of wavelengthsgreater than 5 μm.

In a preferred example embodiment, referring to FIG. 2A, integrationsensor 215 a uses a sapphire window in the front of its housing (TO-5housing). The spectral reception of an integration sensor 215 a with asapphire window and transmission properties of sapphire beam splitter213 are automatically matched. Therefore, integration sensor 215 areceives radiation in the range 0.1-5 μm. Integration sensor 215 b isconfigured with a coated silicon window in front of its housing (TO-5housing) and receives radiation in the range 6.5-20 μm. Integrationsensor 215 b therefore detects most of the radiation which is reflectedoff sapphire plate beam splitter 213. It is not necessary to capture allof the energy reflected by the beam splitter at the long wave sensor,and, therefore, it is more practical and less expensive to employ asensor with a waveband range of about 6.5-20 μm. There is negligibleerror created by allowing the energy of the waveband range of about5-6.5 μm to escape detection.

Radiance W_(λ) is determined at any wavelength λ, as a function oftemperature T, by solving the Planck formula:

W_(λ)=ε_(λ)c₁*λ⁻⁵(exp [c₂/λ*T]−1)⁻¹

Where all constants, including the physical constants, are in CGS units,c₁=3.7413×10⁻¹² and c₂=1.4388. ε_(λ) is the emissivity as function ofwavelength, which depends on surface composition and texture, and isalso function of temperature T.

When the emissivity is constant at two different wavelengths, λ₁ and λ₂,the ratio of radiance, W_(λ2)/W_(λ1), is a function of temperature Tonly. The assumption that emissivity is constant as a function ofwavelength is usually called the “gray body” assumption. If theemissivity at the two wavelengths is not constant, then additionalassumptions are required to determine temperature T from the radianceratio W_(λ2)/W_(λ1). An embodiment of the dual waveband temperaturedetector makes use of integrating sensors to overcome this problem.

In general, the radiances W_(λ1) and W_(λ2) are determined byintegrating over the filter bandwidth of the pyrometer.

W_(λ)=∫_(λ) ₁ ^(λ) ^(u) ε_(λ)W_(λ)∂λ

Where λ_(u) and λ_(l) are the upper and lower band limits, respectively,of each sensor. It is assumed that the wavelength bands of the twosensors are non-overlapping.

A good approximation of the integral can be made by multiplying theradiance at the center wavelength of the filter band by the filterbandwidth ∂λ for narrow band pyrometers, where the relative band widthof the filter ∂λ/λ typically ranges from 0.01 to 0.03. In fact, such anestimation method is used by many providers of narrow band two-colorpyrometers that use micro-computers to calculate the temperature T fromthe ratio of radiances W_(λ2)/W_(λ1).

However, for wide bandwidths, such as 0.1 to 5.0 μm and 6.5 to 20 μm,the estimate is invalid and the integral can be executed to determinethe radiance for each of the two ratio pyrometer sensors. During theconversion of thermal energy to electrical energy, a thermopile executesthe integration of the incident radiation over its bandwidth due to itsrelatively constant absorption of energy at all wavelengths. As such, athermopile is a preferred sensor. If a microcomputer is used for dataevaluation, the integrals can be executed via look-up tables.

Although the narrow wavelength band approximation for radiance isinvalid for determining the radiance over wide bandwidths, a simplifiedapproach to determining radiance exists. The simplified approach makesuse of the Stefan-Boltzmann law that the total energy radiated from asurface is given as:

W=εσT⁴

Where the gray body assumption has been made (ε=constant) and σ is theStefan-Boltzmann constant. This result is equal to the Planck functionintegrated over all wavelengths.

For wide bandwidths, such as 0.1 to 5.0 μm and 6.5 to 20 μm, the Planckintegral can be approximated by

W≈εσT^(x), where x≠4.

Further, if the wavelength band is to the short wavelength side of thepeak value of W_(λ), then x>4. As such, the emitted energy increasesmore rapidly than T⁴. This is the case for bandwidths, such as 0.1 to5.0 μm, employed at common temperatures for industrial use. Forwavelength bands to the long wavelength side of the peak value of W_(λ),then x<4. This means that the emitted energy increases less rapidly thanT⁴ .

Therefore, taking the radiance ratio, and assuming constant emissivity,obtains the following:

W_(λ2)/W_(λ1)≈(ε_(λ2)/ε_(λ1))T^((x2−x1))≈T^((x2−x1))

Experimental data produced by an example embodiment of the presentinvention confirms the approximate power law of radiance ratioW_(λ2)/W_(λ1) as a function of temperature T. Taking the logarithm ofthe radiance ratio approximates a straight line on a semi-log plot ofthe same data for small temperature ranges. Therefore, the radianceratio can be linearly approximated for a narrow range of temperatures,for example, a range of 100 F. If plotted on log-log scale, withabsolute temperature T in Kelvins (K.), the radiance ratio is a linear afunction of temperature T with slope of approximately 1.7, which is thevalue of (x2−x1).

Compared to traditional and currently available ratio pyrometers, whichmeasure the thermal radiance at two narrow wavelength bands andcalculate the temperature from the ratio of these radiances, widewavelength band ratio pyrometers offer several advantages, such asgreater output stability with variations in emissivity, reduction inapparatus cost, improved accuracy for low temperature detection, andproviding for easier and more reliable use.

By integrating (or by utilizing a sufficient estimation technique) theemissivity of a target material over a wide wavelength band, an exampleembodiment of the present invention provides greater output stabilitywith respect to variations in emissivity. Greater stability is providedbecause those variations in emissivity at individual wavelengths, ingeneral, will not affect the overall emissivity over the wide wavelengthband. In other words, the wide wavelength band integration smoothes outthe emissivity variations. Furthermore, because variations in emissivityhave a negligible impact on the overall emissivities ε_(λ2) and ε_(λ1),there is also a negligible impact on the emissivity ratio ε_(λ2)/ε_(λ1).Therefore, under these circumstances, the gray body assumption is notnecessarily required to determine temperature T from the radiance ratioW_(λ2)/W_(λ1).

The teachings of all patents, published applications and referencescited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A temperature detector for detecting the temperature of a targetcomprising: a first sensor that detects radiation of a first wavelengthband; a second sensor that detects radiation of a second wavelengthband; and electronics that provide an output, based on detected signalsfrom the first and second sensors, representing the temperature of atarget, wherein one of the first and second wavelength bands is to ashort wavelength side of a Planck function and the other of the firstand second wavelength bands is to a long wavelength side of the Plankfunction.
 2. The temperature detector as claimed in claim 1 wherein thefirst and second wavelength bands are wideband.
 3. The temperaturedetector as claimed in claim 1 wherein the electronics compute a ratioof the detected signals.
 4. The temperature detector as claimed in claim1 wherein the first and second sensors are thermopiles.
 5. Thetemperature detector as claimed in claim 1 further comprising a beamsplitter.
 6. The temperature detector as claimed in claim 5 whereinwavelengths of less than about 5 microns are primarily passed throughthe beam splitter and wavelengths of greater than about 5 microns areprimarily reflected by the beam splitter.
 7. The temperature detector asclaimed in claim 1 wherein the one wavelength band contains a range ofwavelengths from about 0.5 microns to about 5 microns and the otherwavelength band contains a range of wavelengths greater than or equal toabout 5 microns.
 8. A method of detecting temperature of a targetcomprising: receiving radiation emitted from the target; sensingradiation of a first wavelength band at a first sensor; sensingradiation of a second wavelength band at a second sensor; and providinga temperature output based on the sensed signals from the first andsecond sensors, using electronics, wherein one of the first and secondwavelength bands is to a short wavelength side of a Planck function andthe other of the first and second wavelength bands is to a longwavelength side of the Planck function.
 9. The method as recited inclaim 8 wherein the first and second wavelength bands are wideband. 10.The method as recited in claim 8 wherein the electronics compute a ratioof the detected signals.
 11. The method as recited in claim 8 whereinthe first and second sensors are thermopiles.
 12. The method as recitedin claim 8 further comprising splitting the received radiation byprimarily passing the first wavelength band and primarily reflecting thesecond wavelength band using a beam splitter.
 13. The method as recitedin claim 11 wherein wavelengths of less than about 5 microns areprimarily passed through the beam splitter and wavelengths of greaterthan about 5 microns are primarily reflected by the beam splitter. 14.The method as recited in claim 8 wherein the one wavelength bandcontains a range of wavelengths from about 0.5 microns to about 5microns and the other wavelength band contains a range of wavelengthsgreater than or equal to about 5 microns.